Many of the materials used in the manufacture of semiconductors, optics and a variety of other applications require the highest quality material available to meet the performance requirements expected in the future. This is particularly important when it relates to the quality of the surface, the crystalline structure and the impurities in the material. In the case of semiconductors like silicon and gallium arsenide for instance, a single crystalline defect or impurity near the surface of the material can significantly degrade the performance of an integrated circuit, or keep it from operating at all. Material defects in optics made from silicon, sapphire and special glasses can have catastrophic results when used with high powered lasers or when second order optical effects are being used. These situations have been recognized for some time, and a variety of equipment has been disclosed or developed to measure the surface character of these special materials. For example U.S. Pat No. 4,314,763, entitled "DEFECT DETECTION SYSTEM", discloses one of several techniques used to measure surface defects and contamination on semiconductors.
The measurement of the crystalline and other micro defects directly below the surface, however, has been much more difficult. For example U.S. Pat. No. 4,391,524, entitled "METHOD FOR DETERMINING THE QUALITY OF LIGHT SCATTERING MATERIAL", similar to the one previously mentioned, discloses one technique developed for that purpose. A second approach is described in U.S. Pat. No. 4,352,016, entitled "METHOD AND APPARATUS FOR DETERMINING THE QUALITY OF A SEMICONDUCTOR SURFACE"and U.S. Pat. No. 4,314,017, entitled "APPARATUS FOR DETERMINING THE QUALITY OF A SEMICONDUCTOR SURFACE". All of these measurement techniques have significant limitations when measuring the subsurface crystalline damage or other micro defects which are most important to the improvement and use of these materials.
The term defects, as used herein, refers to any of a variety of structural crystalline defects, either grown-in or processing induced, like slips, dislocations, stacking faults and even buried scratch traces as well as defects which are formed when foreign material is incorporated into the crystal structure such as inclusions, precipitates and impurity clusters and other impurity related defects. Basically there are three ways of generating defects in the materials of interest. First, defects can be grown into the material when it is manufactured in its bulk form. For instance, when single crystals of silicon or gallium arsenide are grown, dislocations can form in the boule due to thermal stresses induced during the growing process, or impurities in the starting material are incorporated into the crystal. Secondly, after the material is manufactured, it must be cut into usable pieces and the surfaces ground and polished in preparation for further processing. These steps of cutting, grinding and polishing also introduce slips and dislocations into the crystal structure, but generally just below the prepared surface. Impurities can also be introduced into the material during these operations by diffusion and other mechanisms. This second class of defects is generally 1,000 to 1,000,000 times greater in number than the defects grown into the original boule of material. Not only are the numbers larger, but as stated, these defects are near the surface while the grown-in defects are distributed throughout the volume of the material. Thirdly, defects such as stacking faults, precipitates, dislocation lines and ion implantation induced defects can be generated by various fabrication processes typically, used in the processing of semiconductor wafers. The same is true of optical materials not only for crystalline defects but also for buried defects which can be generated in amorphous and polycrystalline materials by the preparation processes. These subsurface defects will effect the way light is transmitted through or reflected from an optical material. Another effect, which is just beginning to be understood, is the connection between subsurface defects of all types and coating defects. Since optics and electronics both make extensive use of coatings, such effects are of great importance. For instance, epitaxial layers grown on semiconductor wafers can have stacking faults grown-in during the manufacturing process and these can be related to the defects already existing in the substrate wafer.
One technique currently used to measure crystalline damage is described in U.S. Pat. Nos. 4,362,016 and 4,352,017. This approach measures the reflectance of ultraviolet light, at two wavelengths, from the surface of a semiconductor wafer. This technique is known to be insensitive to damage at any depth in the material primarily because of the use of ultraviolet light which is a shallow penetrator in semiconductor materials. A second factor significantly limiting sensitivity is the reflectance measurement itself. Such measurements are notoriously difficult to make and result in looking for small variations in large numbers, which is one of the reasons why this technique requires measurements at two wavelengths. The practical application of this reflectance technique shows up these deficiencies.
A second approach is described in U.S. Pat. No. 4,391,524. This approach can measure the light scattered from the surface and subsurface regions but because of the geometry of the measurement, important data is lost. There are three factors which bear on this assessment which are independent of the wavelength selected for the probe beam. First, the angle of incidence of the probe beam is 0 degrees. This eliminates any possibility of determining the directional nature of the defects, or of using polarization to help discriminate between surface and subsurface defects. Secondly, the detector subtends a large solid angle thus integrating scatter from all directions, again making impossible the determination of directional defects, and at the same time diluting the signature of the defects it is designed to measure. And finally, the detector line of sight is also at 0 degrees, or near 0 degrees. This introduces significant amounts of surface scatter into the measured signal which is nearly impossible to separate from the subsurface scatter. Subtle variations in surface scatter will mask the scatter from the subsurface that are the purpose of the measurement. The result is a measurement that is insensitive to defect direction and very sensitive to the surface character of the test part.
Other techniques which purport to measure subsurface defects using a scatter measurement technique all measure the total integrated scatter from the surface of the test part. This technique known as TIS integrates the scatter from the surface and subsurface as well as from all directions. The result is a measurement of mostly surface roughness for which this technique was originally designed. The surface scatter component of the total scatter from a material is very large, and will overwhelm the subsurface component if the scattered light is collected anywhere near the specularly reflected beam.